One method to operate an internal combustion engine is described in U.S. Pat. No. 5,682,854. This patent presents a method for changing engine operating modes between Otto cycle operation and Miller cycle operation. The method adjusts intake and exhaust valve opening times in response to requested engine load. At light loads, the valve timings may be set so that the engine operates in an Otto cycle. At higher loads, the valve timings may be set so that the engine operates in a Miller cycle. The inventors of this method claim that it can reduce engine NOx and knock.
The above method also has several disadvantages. Namely, the method changes the engine operating mode based simply on the driver load demand. In addition, the method does not appear to consider that fuel type and/or properties may influence the desired mode of operation. Further, the method reduces the effective compression ratio at higher engine loads and therefore reduces the engine output that may be achieved.